A thought experiment about the Sun vanishing

[Hamiltonian]

TL;DR

if the sun vanishes do you experience loss of gravity first or detect the photons emitted by it before vanishing first?

suppose suddenly the sun disappears at a time t. at this arbitrary time t, the Earth should fling off tangentially to the point in its orbit at time t as there is no centripetal force keeping it in orbit.
we know light takes about 8 minutes to reach the earth.
so will humans on Earth experience being flung off out of orbit first of will the detect the last photons emitted from the sun before disappearing at time t.
if the humans on Earth experience the Earth being flung before detecting the photons hasn't information been transferred faster than the speed of light.

is there something fundamentally wrong with this question because I really can't find an answer.

 

[Ibix]

Using Newtonian physics for this is wrong, because Newtonian physics is a "weak field, everything changes slowly" approximation to general relativity. Something just vanishing violates the "slow change" approximation.

However, general relativity has the local conservation of energy built into its equations. The Sun vanishing violates conservation of energy. So you cannot describe the Sun vanishing in GR either.

So, long story short, as far as we are aware the Sun just vanishing is against the laws of physics we want to use to describe the event. So considering the results of the Sun just vanishing is self-contradictory.

 

[Hamiltonian]

Using Newtonian physics for this is wrong, because Newtonian physics is a "weak field, everything changes slowly" approximation to general relativity. Something just vanishing violates the "slow change" approximation.

However, general relativity has the local conservation of energy built into its equations. The Sun vanishing violates conservation of energy. So you cannot describe the Sun vanishing in GR either.

So, long story short, as far as we are aware the Sun just vanishing is against the laws of physics we want to use to describe the event. So considering the results of the Sun just vanishing is self-contradictory.

ok.
another small clarification.
let's say two masses m1 and m2 are placed a light-year apart. they both exert a gravitational pull on each other suppose the mass m1 is moved by a small amount, will the effect of this change be immediately observed on mass m2?

 

[Ibix]

ok.
another small clarification.
let's say two masses m1 and m2 are placed a light-year apart. they both exert a gravitational pull on each other suppose the mass m1 is moved by a small amount, will the effect of this change be immediately observed on mass m2?

Exactly what, if any, change there is is not trivial to determine. The problem with casually moving a mass is the conservation of momentum - to move the mass you have to put something in motion in the opposite direction, and that will have a gravitational effect too. However, we can guarantee that the gravitational field at m2 will not change before light from m1 starting to move reaches m2, if that's what you are asking.

 

[davenn]

... will the effect of this change be immediately observed on mass m2?

Gravity and light propagate at the speed of light

so that answers both your, Earth - Sun and your M1 - M2 situations

 

[phinds]

is there something fundamentally wrong with this question because I really can't find an answer.

And, did you try just Googling "propagation speed of gravity?

 

[berkeman]

suppose suddenly the sun disappears at a time t. at this arbitrary time t, the Earth should fling off tangentially to the point in its orbit at time t as there is no centripetal force keeping it in orbit.

Or maybe other people just look at you funny and fling you off the Earth when you say you are worried that the Sun keeps disappearing for you...

https://giphy.com/embed/height=500;id=l1J9NeoLE9gYPi1Xi;type=gif;width=500

 

[DaveC426913]

Gravity and light propagate at the speed of light

At the risk of being persnickety , but to steer the OP in the right direction...

Gravity has "always" been everywhere. Changes in gravity propagate at the speed of light.

 

[sophiecentaur]

Summary:: if the sun vanishes do you experience loss of gravity first or detect the photons emitted by it before vanishing first?

is there something fundamentally wrong with this question because I really can't find an answer.

This is not a totally silly question because it is only just unreasonable - being on the fringe of a number of much more reasonable scenarios. If you are just asking about Gravity waves then there is a perfectly good answer (=c). But if you actually want an example of a process that generates them then there are dozens. (But remember it's a very small force to detect). There's nothing much you can do with a spherical object to get gravity waves from it that doesn't involve something else to cause the effect. There will be a changing gravitational field as an object changes its distance from you but the delay, compared with the object's speed would be very very tiny. (Those waves would be more like the tides than the waves on the ocean.)There are other matters, discussed above.

Otoh, an object like a dog bone (or a binary star) will have varying Gravitational Potential, with respect to an observer as it spins from end-on to side-on). You could expect to detect the variations in g field as two stars orbit one another (Much bigger waves for two black holes orbiting) and they would coincide in frequency with the variations of any observed optical magnitude - showing that come from the same place. This link about Ligo is worth reading.

 

[davenn]

Gravity has "always" been everywhere. Changes in gravity propagate at the speed of light.

well if you want to be that picky

All changes in the EM field ALSO propagate at the speed of light

 

[phinds]

All changes in the EM field ALSO propagate at the speed of light

As do comments by all us nitpickers here on PF

 

[Nugatory]

As do comments by all us nitpickers here on PF

Those actually move at speeds faster than light, but there is no violation of relativity because they transfer no information.

 

[fresh_42]

The question has been answered scientifically (impossible) and in the sense of the OP (c is top speed for light and gravity). As "what if" questions tend to become humorous, I think we can close this thread.